GM definitely tests their vehicles out here, including Cadillac. As for Mercedes, I can report that they've definitely tested an AMG version of the E coupe, which has sadly not yet come to fruition (not that I could afford it).

I have seen more mules out here than I could count, but immediately jumping to mind are fleets of Audi, BMW (Stay at the Holiday Inn in Frisco, CO), Dodge/Chrysler/Fiat, Honda, Mercedes, Ford, GM of varying brands (definitely Chevy and Caddy), Isuzu, and I know I'm forgetting a couple. I'm sometimes amused that they camo some of the cars because it often draws more attention to them than if they just left them in plain skin.

You are right. It''s funny how they go to all that work to hide minor styling cues so the competion doesn't know what they are doing. Some of the styling changes are as subtle as making the rear side windows curl up, curl down or cut down at a sharp angle.